In the production of capacitors, anodes based on niobium or tantalum are frequently used. These often consist of specially shaped sheets of said metals to which a paste containing niobium powder or tantalum powder is applied, for example by means of screen printing. The paste is then subjected to a sintering step, with a sintered body which is firmly connected to the shaped article of niobium or tantalum being formed.
The required shaped articles of niobium or tantalum can be etched, for example, out of niobium or tantalum sheets of suitable thickness.
It is known that niobium and tantalum can be etched both by plasma methods and other physical methods and by wet chemical methods. In the physical methods, however, the etching rates are very low. Structures of several 10 μm thickness, as are required for the production of shaped articles, therefore, cannot be produced in acceptable times by these methods. Moreover, the high costs for the equipment and the operation of units, as required for carrying out physical etching methods, may be mentioned as a further disadvantage.
It is therefore economically expedient to use only wet chemical methods for etching shaped articles from niobium or tantalum. Both purely chemical and electrochemical methods are suitable. Before the etching process, a photosensitive resist, a so-called photoresist, is applied to the niobium or tantalum layer to be etched. By suitable exposure to light, the photoresist can then be structured. The photoresist can then be removed, for example by washing out, in the parts where the etching process is to take place, so that the niobium or tantalum layer underneath can come into contact with an etching solution in these parts. The regions of the niobium or tantalum layer in which the photoresist is not removed are protected by this coating from attack by the etching solution.
Methods in which no mask need be produced from a photoresist, such as ECM (electrochemical machining) and the corresponding micro method, EMM (electrochemical micromachining), are also known. In these methods, the selective dissolution in predetermined parts of the workpiece is achieved by the very close approach on an appropriately shaped cathode to the workpiece. In the gap of a few 10 μm thickness between workpiece and cathode, very high current densities and accordingly also high etching rates can be achieved by intensive feeding of the etchant and virtually perpendicular etched edges can be produced. This method, however, requires suitable tools in the form of cathodes, which must be extremely resistant to etching for the application described. The production of such tools, which are manufactured individually from expensive materials, is very costly. This also applies to the required apparatus which generates the necessary intensive flow.
For the electroless wet chemical etching of niobium and tantalum, mixtures of hydrofluoric acid or fluorides with other acids and oxidizing additives, predominantly nitric acid, are generally described. The oxidizing agent has, inter alia, the task of preventing the formation of gaseous hydrogen which can adversely affect the adhesion of a mask comprising a photoresist on the metal.
The metals niobium and tantalum have a dense, firmly adhering, chemically very inert oxide layer and are therefore insoluble in the customary acids, with the exception of hydrofluoric acid. The oxide layer also makes it impossible to establish electrochemical potentials in a reproducible manner, so that very different data are to be found in the literature. However, normal potentials which were estimated from thermodynamic data are in the substantially negative range close to −1 V for niobium and tantalum. These metals can therefore be dissolved purely chemically in hydrofluoric acid without addition of oxidizing agents, but the resulting overvoltage of the hydrogen forming retards the dissolution and hence the chemical etching process.
The chemical etching of niobium and tantalum is as a rule therefore used only when the thickness of the layer to be etched is only a few micrometers. Such thin layers can still be completely etched in an acceptable time. Etching times of a few seconds are sufficient, so that conventional photoresists which are as a rule applied to the niobium or tantalum layers to be etched are not attacked.
U.S. Pat. No. 4,266,008 describes the production of superconducting circuits. For this purpose, a mask of photosensitive material is applied to thin niobium films having a thickness of from 0.5 to 5 μm and the niobium film is then etched in the parts which are not covered with the material, by means of an aqueous etching solution. The etching solution contains from 8.5 to 9% by weight of nitric acid, from 11.5 to 12% by weight of sulphuric acid and from 12.25 to 12.75% by weight of hydrofluoric acid. A 0.5 μm thick niobium layer can be completely etched with this etching solution in 10 s.
JP 56081680 A, too, discloses a process for etching niobium. A mixture of from 5 to 15% by weight of HF, from 2 to 6% by weight of NH4F, from 10 to 20% by weight of HCl and from 3 to 9% by weight of HNO3 is proposed.
M. Köhler in “Ätzverfahren für die Mikrotechnik [Etching methods for microengineering], WILEY-VCH-Verlag, 1998, page 306” describes an etching bath composed of 0.66 mol/l of (NH4)2S2O8, 0.27 mol/l of NH4F, 0.11 mol/l of citric acid and 1.43 mol/l of HNO3. The etching process is carried out at a temperature of 50° C.
In “Metallographisches, keramographisches, plastographisches Ätzen [Metallographic, ceramographic, plastographic etching], Gebr. Borntraeger Verlag, 6th edition, Stuttgart 1994, page 99”, G. Petzow provides information on the microetching of niobium and its alloys with an etching solution which contains no fluorine compound. The etching solution consists of a mixture of equal parts of water, 30% strength by weight hydrogen peroxide and 32% strength by weight ammonia solution and is intended to be used at the boiling point. Since such a mixture is stable for only a short time, in particular at the boil, it is not suitable for the production of shaped articles of niobium and tantalum by means of etching.
The two metals niobium and tantalum can be etched not only with hydrofluoric acid but also with strongly alkaline solutions, to which, however, all known photoresists are insufficiently stable. Alkaline etchants are therefore not suitable for chemical machining. Owing to the evident difficulties due to the high acid concentrations and the necessity of adding oxidizing agents, the purely chemical methods for the production of shaped articles of niobium and tantalum are not very promising.
In principle, no oxidizing agents are required in electrochemical etching. Nevertheless, G. Petzow in “Metallographisches, keramographisches, plastographisches Ätzen [Metallographic, ceramographic, plastographic etching], Gebr. Borntraeger Verlag, 6th edition, Stuttgart 1994, page 99” specifies a mixture of 17 parts of 65% strength by weight HNO3, 17 part of 40% strength by weight HF and 66 parts of water for such a process. According to G. Petzow, the etching is effected with from 12 to 30 volt against a Pt cathode. In the electrochemical etching of niobium, no passivation, as occurs in the case of some metals in certain etchants, was described in the case of the anodic dissolution in HF. In the electrochemical etching of tantalum at a current density greater than 1 A/dm2, on the other hand, a layer of H2TaF7 forms, which layer hinders the further dissolution and increases the bath voltage.
A disadvantage of the wet chemical methods described is the intensive attack by the etching solutions used on the photoresist mask, which prevents destruction of niobium and tantalum layers having a thickness substantially greater than 10 μm. However, shaped articles of a corresponding thickness are required, for example, for the production of niobium or tantalum anodes. A further disadvantage is that single crystallites are exposed with the use of the etching solutions described, which results in a very rough surface in the etched parts.
It is therefore an object of the present invention to provide a process for etching shaped articles of niobium and tantalum which does not have said disadvantages and in particular permits the production of shaped articles having a thickness greater than 50 μm.